1. Field of the Invention
The present invention relates to radiation-curable resins based on ketone-aldehyde and to urea-aldehyde resins and to a process for preparing them.
2. Description of the Related Art
Radiation-curable coating materials have increasingly gained importance within recent years, due to, for example, the low VOC (volatile organic compounds) content of these systems.
The film-forming components in the coating material are of relatively low molecular mass and hence of low viscosity, so that there is no need for high fractions of organic solvents. Durable coatings are obtained by the formation, following application of the coating material, of a high molecular mass, polymeric network by means of crosslinking reactions initiated, for example, by UV light.
Ketone-aldehyde resins are used in coating materials, for example, as additive resins in order to enhance certain properties such as initial drying rate, gloss, hardness or scratch resistance. Due to their relatively low molecular weight, customary ketone-aldehyde resins possess a low melt viscosity and solution viscosity and therefore also serve as film-forming functional fillers in coating materials.
Ketone-aldehyde resins normally possess hydroxyl groups and can therefore be crosslinked only with, for example, polyisocyanates or amine resins. These crosslinking reactions are usually initiated and/or accelerated thermally.
For radiation-initiated crosslinking reactions, in accordance with cationic and/or free-radical reaction mechanisms, the ketone-aldehyde resins are not suitable.
Accordingly, the ketone-aldehyde resins are normally added to radiation-curable coating systems as, for example, a film-forming component, but not as a crosslinking component. Due to the uncrosslinked resin fractions, the resistance of such coatings to gasoline, chemicals or solvents, for example, is often relatively low.
WO 95/17476, DE 23 45 624, EP 736 074, DE 28 47 796, DD 24 0318, DE 24 38 724, and JP 09143396 describe the use of ketone-aldehyde resins and ketone resins, e.g., cyclohexanone-formaldehyde resins, in radiation-curable systems. Radiation-induced crosslinking reactions of these resins are not described.
EP 0 902 065 describes the use of nonradiation-curable resins formed from urea (derivatives), ketone or aldehydes as an added component in a mixture with radiation-curable resins.
DE 24 38 712 describes radiation-curing printing inks composed of film-forming resins, ketone resins and ketone-formaldehyde resins, and polymerizable components such as polyfunctional acrylate esters of polyhydric alcohols. To the skilled worker it is obvious that radiation-induced crosslinking reaction of the modified ketone-aldehyde resins and ketone resins can only come about through the use of unsaturated fatty acids. It is known, however, that resins having a higher oil content tend toward unwanted yellowing.
U.S. Pat. No. 4,070,500 describes the use of nonradiation-curable ketone-formaldehyde resins as a film forming component in radiation-curable inks.